In a large skillet, brown the onion and mushrooms in the butter and oil. Season with salt and pepper. Sprinkle with the flour and stir to blend. Stir the broth, wine and cream into the mushroom mixture and bring to a boil. Simmer gently for about 3 minutes. Add the asparagus and duck meat. Reheat and adjust the seasoning. Fill the vol-au-vents.